1. Field of the Invention
The present general inventive concept relates to an apparatus to remove air bubbles in ink to be supplied to an inkjet printer head of an inkjet printer and a method of removing the air bubbles, and more particularly, to a bubble removing apparatus that can be applied to an array head of an inkjet printer that uses a line printing method, and method of removing the air bubbles using the same.
2. Description of the Related Art
Conventionally, an inkjet printer prints a desired image on paper by ejecting ink droplets onto the paper. As illustrated in FIG. 1, an inkjet printer is formed as part of a conventional image printing apparatus and includes a printer head 10 that ejects ink droplets through nozzles 11a, a pair of feed rollers 21 that push a paper P, received from a front-end portion 15 of a conventional image printing apparatus, under the printer head 10, and a pair of discharge rollers 22 that discharge the paper P to a tray 30. The front-end portion 15 may include a scanning unit to scan an image and a paper storage unit to store blank sheets of paper (not illustrated). When the feed rollers 21 push the paper P under the printer head 10, the printer head 10 prints a desired image by ejecting ink droplets through the nozzles 11a of a chip 11, and the discharge rollers 22 push out the paper P on which the desired image is printed to the tray 30.
Printing methods include a shuttle method in which an image is printed on a paper P in a horizontal writing method while the printer head 10 reciprocally moves back and forth over the width of the paper P, and a line printing method in which a fixed printer head 10 is formed to cover the whole width of the paper P and an entire line of the image is simultaneously printed. Recently, the line printing method, that is, an array head is widely used due to its high printing speed.
In the inkjet printer described above, since an image is printed by ejecting ink droplets through the nozzles 11a formed in the chip 11 of the printer head 10, if the nozzles 11a are blocked by air bubbles, ink cannot be properly ejected, thus the image cannot be accurately printed. In order to avoid this problem, various methods to remove air bubbles in the ink have been proposed. A method commonly used is sucking out the air bubbles present inside the nozzles 11a together with a small amount of ink using a pump after covering a suction cap on the chips 11 of the printer head 10. This method is effective in a shuttle method head having small number of chips 11 and a relatively small area. However, in the case of an array head operating in a line printing method in which the chips 11 are widely disposed almost to cover the entire width of the paper P, it is difficult to apply the suction method. That is, in order to cover the entire width of the paper P, a lot of chips 11 having nozzles 11a must be disposed in the widthwise direction of the paper P. In this case, it is difficult to seal each of the entire chips 11 with a cap and to suck out air bubbles by applying a uniform pressure to all of the nozzles 11a. In the case of the array head, in order to address this problem, as illustrated in FIG. 2, an ink circulation line 60 is formed between the printer head 10 and an ink tank 50 and ink in the printer head 10 is circulated by operating a pump 70 when necessary. That is, air bubbles are induced into the ink tank 50 by periodically circulating the ink in the nozzles 11a and the air bubbles are separated by gravitational difference. Reference numeral 40 indicates a negative pressure generator that maintains the pressure in the nozzles 11a of the chip 11 at a negative pressure, and reference numeral 90 indicates a filter unit for filtering foreign materials included in the ink. Thus, the ink in the ink tank 50 is supplied to the printer head 10 to be used for printing work by the pump 70, and in this process, any foreign materials included in the ink are filtered by the filter unit 90. When it is necessary to remove air bubbles, a valve 80 is opened to circulate the ink. Thus, air bubbles are collected in the ink tank 50, and are separated by the gravitational force.
As illustrated in FIG. 3, a filter unit 90 having a vertical structure is widely used, in which a filter 92 is vertically mounted in a housing 91 having an ink inlet 91a and an ink outlet 91b. That is, ink that enters the filter unit 90 through the ink inlet 91a is filtered while passing through the filter 92, and then, is supplied to the printer head 10 through the ink outlet 91b. 
As illustrated in FIG. 3, air bubbles B that enter the housing 91 of the filter unit 90 are held in the housing without passing through the filter 92. That is, the air bubbles B typically enter when the ink circulation line 60 is refilled after the ink tank 50 is replaced. The air bubbles B that have entered the filter unit 90 in this way cannot pass through a mesh of the filter 92 and are held on the side of the ink inlet 91a of the housing 91. In this case, due to the air bubbles B, an area of the filter 92 to be used is reduced, thereby increasing pressure loss. That is, this situation is equivalent to the case where the cross-sectional area of a pipeline is reduced, and thus, a pressure drop occurs at the filter unit 90. Accordingly, a stable ink supply to the printer head 10 cannot be achieved resulting in the poor printing quality. In order to determine the trend of pressure loss when a working area of the filter 92 is gradually reduced, an experiment was performed. FIG. 5 is a graph showing the measurement result of pressure drop at the boundary of an ink inlet and ink outlet of a housing when filters having different diameters from each other are used. Filters respectively having a diameter of 10 mm, 20 mm, 30 mm, and 40 mm were used in order to correspond to the situation where a filter having a diameter of 40 mm is used at first and the diameter of the filter is then gradually reduced to 30 mm, 20 mm, and 10 mm due to the air bubbles. The size of mesh is unchanged. From the graph, it is seen that as the working area of the filter is gradually reduced, the pressure drop rapidly increases.
In order to address this problem, a method of increasing the speed of fluid can be employed. That is, a large amount of ink is rapidly passed through the filter 92 so as to allow the air bubbles to penetrate through the mesh without being held in the filter unit 90. However, when the flow speed of the ink is increased, a negative pressure at the nozzles 11a of the printer head 10 is increased. Thus, external air can enter the printer head 10 through the nozzles 11a, and the external air can be a source of air bubble generation. Therefore, increasing the flow speed of ink is not a desirable solution.
As illustrated in FIG. 4, there is a structure in which a filter 12 is horizontally installed on a front end of a chip 11 in the printer head 10. In this structure, air bubbles B generated during ejection of ink through the nozzles 11a float and gather at an ink outlet side of the filter. Also, as it is well known in the art, a heater (not shown) is installed in the printer head 10 corresponding to each of the nozzles 11a. Ink droplets are ejected through the corresponding nozzle 11a by small air bubbles generated due to heat of the heater. The small air bubbles generated at this point rise to the filter 12 and cannot pass through the filter 12. In this case also, the air bubbles interrupt the ink from flowing towards the nozzle 11a. For example, when the heater is operated in a state where the ink is not filled inside the filter 12, the lifetime of the printer head 10 can be greatly reduced due to overheating.
Therefore, to address the above problems, there is a need to develop a method of smoothly removing air bubbles embedded in ink.